The present invention relates generally to transthoracic impedance and more particularly to a method for determining airway obstruction using transthoracic impedance.
Studies of Sudden Infant Death Syndrome (SIDS) tracings indicate three phases in the death of infants—hypoxic hyperpnea, apnea and gasping. Hypoxic hyperpnea is an abnormal increase in the depth and frequency of breaths resulting in a drop in oxygen content of air in the lungs. Apnea is a temporary cessation of breathing, and gasping is a uniquely regulated type of breathing that occurs under conditions of severe brain hypoxia. Autoresuscitation, which is defined as an increase in heart rate following a gasp, occurs less often in SIDS infants than infants dying due to other causes. Because successful lung inflation during gasping is the primary mechanism for autoresuscitation, an obstructed airway can prevent autoresuscitation potentially resulting in death.
There are two critical periods in the sudden death of infants. A first period includes factors precipitating hypoxia (i.e., a drop in oxygen content of tissue in the lungs) and a second period is a failure of gasping to alleviate hypoxia. Many SIDS infants have obstructed airways contributing to the initial hypoxia, as well as to the failure of autoresuscitation during hypoxic gasping.
Transthoracic impedance, which is a measure of the electrical impedance across an individual's thorax (i.e., the cavity in which the lungs and heart are positioned), is commonly used to monitor respiration (i.e., breathing). Because air is a relatively poor conductor of electrical current and blood is a relatively good conductor of electrical current, transthoracic impedance is an indication of the ratio of air to blood in the thorax. As the amount of air in the thorax increases, transthoracic impedance increases. Conversely, as the amount of blood in the thorax increases, transthoracic impedance decreases. Even though transthoracic impedance is an indication of the ratio of air to blood, it is not a direct measure of breathing because transthoracic impedance continues to oscillate even when the airway is obstructed. For this reason, in the past it was believed that transthoracic impedance could not be used to detect obstructive apnea because the impedance signal continues during airway obstruction.
Other methods have been explored for determining when a breath is obstructed. For example, a reduction in the sum channel of respiratory inductance plethysmography has been used to identify obstructed breaths. However, this method only detects about 54% of obstructions when compared to end tidal carbon dioxide measurements. Thus, this method is not an effective way of indicating airway obstruction. Because most infant apneas consist of obstructed breaths occurring during periods of absent respiratory efforts, the total duration of an apneic event is not documented by current methods and the monitor may not alarm in time for a caretaker to use cardio pulmonary resuscitation. Furthermore, healthcare professionals may be aware an infant is having prolonged apneic events and discontinue monitoring. Accordingly, a need exits for an effective indicator of airway obstruction to reduce occurrences of death from prolonged apneic events.